Mayorga-Adame,C. G., Sanga,I. P. L., Majuto, C., Makame, M. A., Garu,M

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Training program on Modelling: A Case study – Hydro-dynamic Model of Zanzibar channel

  • Mayorga-Adame,C.G., Sanga,I.P.L.,

  • Majuto, C., Makame, M.A., Garu,M.


  • Hydrodynamic Modeling

    • In understanding the marine and coastal environment the contribution of hydrodynamic numerical models become highly indispensable.
    • Water quality, sediment transport and ecology models all rely heavily on the results of the hydrodynamic simulations (water levels, horizontal & vertical velocities, salinity, temperature, density pattern etc).

    • The main goal of the hydrodynamic modeling is to provide the state of the art and accurate simulation for seeking the solutions for challenges present in marine and coastal environment.


  • Research institutions such as IMS, TAFIRI

  • Commission of Tourism (for management of beaches and hotels along the coast)

  • Port Authorities (Dsm & Zanzibar)

  • Department of environment

  • Department of fisheries (egg and larval transport)

  • Municipal councils

  • Department of lands

  • Oil exploration (for rigs)

  • Other scientist and stakeholders

  • Local communities

What is ROMS?

  • The Regional Ocean Model System (ROMS) is a free-surface model that solves the hydrostatic primitive equations.

  • It uses stretched, terrain-following coordinates in the vertical and orthogonal curvilinear coordinates in the horizontal.

  • It was developed by Rutgers University and is broadly used in sciences worldwide.


  • It is free access software.

  • It is more realistic than other models:

    • It is a 3 dimensional model.
    • Includes complex ocean dynamics.
      • Coriolis effects
      • Friction
      • Heat fluxes
      • Vertical Mixing
    • Atmospheric and oceanographic features can be included:
    • Biological systems and sediment transport dynamics can be coupled to the hydrodynamic model.

Why ROMS? (Cont.)

  • It allows you to assess the different physical processes that are going on in the ocean, individually.

  • Results are quite accurate, and reproduces the most important features of ocean circulation and hydrodynamics of the study area even using a semi-idealized application.

The difficult of using ROMS

  • It is as complex as the ocean itself…

    • It runs in Linux operating system.
    • It doesn’t have a graphical user inter-phase.
    • It can not be used as a black box or a plug and play model.
    • A deep understanding of ocean dynamics and modeling is needed to configure the model.
    • Knowledge about the local oceanographic features affecting the area of study are also required.
    • It doesn’t work with default values. Real data is required.
    • Format of input and output files. NetCDF (binary files).
    • Skills in different software are needed for data treatment and results analysis (matlab, Linux, fortran, etc).


  • To build capacity of modeling team at IMS.

  • To develop a Local Semi-idealized Model of Zanzibar Channel.

Case of study

  • Zanzibar channel located between longitude 38.8oE – 39.6oE and latitude 5.5oS – 6.8oS. (the coasts of Tanzania mainland and that of Unguja Island)

  • The length of the channel is about 120Km and 35 – 40Km wide


  • Data collection:

      • Bathymetry and coast line data from global data bases.
      • Local bathymetry data
      • Wind
      • Tides
      • Oceanographic parameters
      • Current (for validation of the model).
  • Data analysis and processing.

  • Preparation of input files

  • Configuration ROMS for the Zanzibar Channel.

  • Analysis of the results and validation of the results.

Data analysis and processing

  • Local bathymetry data.

    • Direct measurement (Dr. Shagude)
    • GIS laboratory data base (Kombo)
    • Digitize from nautical charts (Dr. Dubi)

Data analysis and processing (cont.)

  • Winds

    • Monthly averages of wind data (speed and direction) for 10 years (1996-2005) from Zanzibar airport meteorological station.

Data analysis and processing (cont.)

  • Tide

    • Topex/Poseidon global co-tidal map for the component M2 shows that the tide is coming in through the southern mouth of the channel.

Data analysis and processing (cont.)

  • Oceanographic parameters

    • CTD casts near the Zanzibar Channel from a Western Indean Ocean Oceanographic Cruiser (Dr. Shaghude).

Preparation of input files

Preparation of input files (cont.)

Configuration of ROMS for the Zanzibar Channel.

  • 60 x 100 cells covering 72.4 x 88.4 km

  • 16 levels in the vertical.

  • Resolution ~ 1km2

  • Minimum depth 2 m.

  • Maximum depth 66 m

  • Time step 100 sec.

  • North and South boundaries open.

  • Oceanographic parameters

    • Salinity 34.9 oo/o
    • Temperature 26 o C
    • Density 1023 Kg/m3

Configuration of ROMS for the Zanzibar Channel (cont.)

  • Analytical Forcing

  • Wind

    • It was establish as a surface momentum flux uniform over all the domain.
    • The wind stress was calculated based on the magnitude of the wind velocity components.
    • A linear ramp was use to increase wind magnitude from zero to its maximum during the first 2 days modeled, after this wind remain constant over time.
    • The two more common wind condition in the zone were reproduced SE and NE winds.
  • Tide

    • Was establish as a sea level perturbation at the South open boundary, with the frequency of the M2 component (12.42 hrs), and an amplitude of 1 m. The model it self propagate the perturbation Northward along the domain.

Running the model

  • We run the model until it gets to stable state. The intermediate states skipped “spin up” that considered to be not valid results.

    • 10 days of simulation for wind = 1hr and 35 min computing time
    • 5 days of simulation for tide = 20 min computing time


  • SE wind (July)


NE wind (January)

  • NE wind (January)


  • NE wind (January)

Further steps…

  • Putting all together wind, tide, stratification, and running for long time.

  • Including more local data if available.

  • Validating the results.

  • Developing a mesoescale model with global data to get more realistic forcing fields for the free surface and open boundary conditions.

  • Nest the local model to the mesoescale one.


  • We have

    • A team that is doing well and a building capacity of modeling at IMS will be achieved
    • Developed a hydrodynamic model of Zanzibar channel. It revealed a number of observations
  • To make a useful model for the stakeholders for use in making decisions

    • We need to incorporate local measured data of the channel (salinity, temperature, current, etc).
    • Global data on the Zanzibar channel is not available.
    • The model should be run for long time.
    • We also need to validate the model.
    • The model is very potential and it can be very useful for management, planning and decision making

Thank you

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